1. Field of the Invention
The present invention relates to an electrostatic actuator.
2. Discussion of Background
An electrostatic actuator is mainly composed of a stationary element formed by arranging electrodes with a predetermined distance on an insulating supporter and a movable element formed by disposing a resistance layer on an insulating sheet such as an insulating film wherein the stationary element and the movable element are in contact with each other. The electrostatic actuator is adapted to instantaneously raise the movable element by the function of static electricity and move the movable element without any frictional force (vide, a proceeding number 6/191 in national session of 1989's The Institute of Electrical Engineers of Japan, and Japanese Unexamined Patent Publication No. 285978/1990).
The electrostatic actuator has such features that a density of force can be increased by reducing the dimensions of the electrodes and electrode gap, and the size can be made small. Accordingly, the electrostatic actuator is expected to be used as a small-sized driving device such as a paper transferring mechanism for a word processor, a facsimile machine or the like and a driving device for another small mechanical system.
FIGS. 7a through 7d are diagrams showing the principle of operation of a conventional electrostatic actuator (an electrostatic film actuator) in which a movable element comprises an insulating film. In the Figures, reference numeral 1 designates an insulating supporter, numeral 2 designates strip electrodes, numeral 3 designates a stationary element, numeral 4 designates an insulating film, numeral 5 designates a resistance layer, numeral 6 designates a movable element and numerals 7, 8 and 9 designate electric wires.
In operation, first, a positive voltage and a negative voltage are applied to wires 7 and 8 respectively as shown in FIG. 7a. Then, a current flows in the resistance layer 5 due to a potential difference between electric charges 1 at the electrode connected to the wire 7 and electric charges 2 at the electrode connected to the wire 8. The electric charges are induced at the boundary of the insulating film 4 and the resistance layer 5 of the movable element 6 to reach an equilibrium state. For convenience of explanation, the induced electric charges are represented by electric charges 3 and 4 having opposite polarities shown by dotted lines in FIG. 7b. In the state shown in FIG. 7b, since the polarities of the electric charges 3 and 4 are respectively different from the polarities of the electric charges 1 and 2, the movable element 6 is attracted to the stationary element 3.
Then, a negative voltage is applied to the wire 7, a positive voltage is applied to the wire 8 and a negative voltage is applied to the wire 9 as shown in FIG. 7c. Then, the electric charges at the electrodes can be instantaneously changed, however, the electric charges having the opposite polarities in the movable element 6 can not be immediately moved because the resistivity in the resistance layer 5 is high. As a result, a repulsive force takes place between the movable element 6 and the stationary element 3. The repulsive force reduces friction between the stationary element 3 and the movable element 6, and a driving force in the right direction in the Figure is produced due to the negative electric charges 5 and the induced electric charges 4 (in terms of the electric charges having the opposite polarity) as a result of the application of the negative voltage to the wire 9.
FIG. 7d shows a state that the movable element 6 is shifted in the right direction by one pitch of electrode by the driving force. When the movable element 6 is to be moved in the left direction, a positive voltage is applied to the wire 9.
In a voltage application pattern (FIG. 7c) in the operations to move the electrodes by one pitch, voltages having opposite signs with respect to the state shown in FIG. 7a are applied to the wires 7 and 8. Therefore, the induced electric charges (3 and 4 in terms of the electric charges having opposite polarities) attenuate.
Accordingly, in order to successively move the movable element 6 in the right direction by one pitch of electrode, it is necessary to repeatedly apply the voltage pattern, as indicated below, i.e., it repeats a charging operation of electric charges and a moving operation. The voltage pattern, described below, shows a voltage pattern for one cycle wherein a symbol G represents a non-voltage application state (an earthing state), a sign (+) represents a state of applying a positive voltage, and a sign (-) represents a state of applying a negative voltage. Characters C and A represent the charging operation and the moving operation respectively, a character C1 represents the state shown in FIG. 7a, and a character A1 at the first represents the state shown in FIG. 7c.
______________________________________ C1 A1 C2 A2 C3 A3 (one cycle) ______________________________________ Wire (7) + - G - - + Wire (8) - + + - G - Wire (9) G - - + + - ______________________________________
When an arrangement of electrodes of a three phase structure is used, for instance, the pattern for driving voltage may be selected from various types of voltage pattern as far as they cause the charging operation and the moving operation repeatedly in which the earthing state is provided at a suitable timing. For instance, a voltage pattern in which C1 and C2 are omitted can be used.
In order to successively move the movable element in a stable manner for every one pitch of electrode, it is said that the surface resistivity of the movable element 6 (or the resistance layer 5) be in a range of 10.sup.11 -10.sup.15 .OMEGA./.quadrature.. The reason is as follows.
When the surface resistivity of the movable element 6 is too high, it takes relatively long time for charging electricity. On the other hand, when it is too low, electric charges attenuate instantaneously, so that it is difficult to successively move the movable element in a stable manner.
In the electrostatic actuator shown in FIG. 7, the resistivity of the insulating film 4 forming the movable element is too high. Accordingly, it is necessary that the resistance layer is formed on the insulating film so that the resistance layer has a slight electric conductivity by the reason as described above. Further, in place of the insulating film 4, another insulating sheet having the same resistivity may be used.
At present, the electrostatic actuator has been in study, and the various structural elements have to be examined in detail in order to render the actuator to be practical use. In particular, since the stationary element and the movable element of the electrostatic actuator are formed of a sheet-like body, it is easy to reduce the size and the thickness. However, it is necessary to reduce the size and the thickness of a driving device for moving the movable element in order to use the electrostatic actuator practically.
Further, cost of the driving device should be reduced in order to accelerate the practical use of commodities to which the electrostatic actuator is applied. Although some published documents describe the study of the principle of driving the electrostatic actuator and a certain achievement, they fail to disclose the study of reducing the cost.
In particular, since a high voltage such as more than 100 V has to be used as a driving voltage for the electrostatic actuator, cost for a high voltage power source of direct current and a high voltage switching circuit is pushed up, and therefore, it is expected to reduce the cost of these parts.
On the other hand, the study of reducing the magnitude of driving voltage is made to reducing the cost. For instance, if the distance between the electrodes of the stationary element can be reduced to several tens .mu.m, the driving voltage can be reduced. However, the idea is not practical at the present stage since there is a demand of increasing the surface area of the electrostatic actuator.
In "Articles of Rotating Machine Society" (published by The Institute of Electrical Engineers of Japan on Nov. 15, 1991), use of a piezoelectric transformer as a high voltage power source for the electrostatic actuator is proposed. However, the piezoelectric transformer suffers restriction in practical use since the output current is extremely small as described in comparative example described before.
The inventors of this application have made intensive study in view of the above-mentioned situation, and have obtained the knowledge as follows.
The electrostatic actuator can be driven by repeating charging and discharging operations to a load having a relatively small capacitance such as about 100 pF-10 nF. Although a high voltage is necessary for moving the movable element, an amount of electric current can be small as several mA or lower. Further, the durability of the electrodes on the stationary element is improved by reducing the amount of electric current from a high voltage power source of direct current.
The equivalent circuit per unit of the electrostatic actuator can be expressed by the capacitance C and the resistance R formed by the stationary element and the movable element. A symbol R1 indicates the resistivity of the movable element, and the surface resistivity is generally in a range of 10.sup.11 -10.sup.15 .OMEGA./.quadrature. as described before. A symbol R2 indicates the resistivity of an insulating material for forming an insulating layer on the surface of the electrodes in order to improve an insulation property between the electrodes.
An electrostatic energy for driving the movable element is expressed by the product of the square of the capacitance C and an applied voltage, and an electric current flowing in the insulating material having a resistance R2 between the electrodes becomes loss. When the electrostatic actuator is driven in the state shown in FIG. 7, a substantial amount of current flows in the insulating material having a resistance R2 between the electrodes. Accordingly, the loss of electric current can be reduced by selecting material having a large volume resistivity such as 10.sup.11 .OMEGA. cm or higher. Further, it is possible to minimize a consumption current to drive the electrostatic actuator.
On the other hand, when the insulating material between the electrodes is moisturized or pin holes takes place in it whereby the insulation strength decreases, a minute electric discharge may happen between the electrodes.
Further, when the capacity of electric current of the high voltage power source of direct current is large, electric discharge may gradually spread to thereby cause breakage of the electrodes. Accordingly, when the capacity of electric current of the high voltage power source of direct current is made small to an extent of capable of driving the electrostatic actuator, the size of the high voltage power source can be small, and at the same time, the spreading of electric discharge can be prevented, whereby the durability of the electrodes can be improved.